1. Field of the Invention
The present invention relates to a flexible wiring board, and to a liquid discharge recording head unit using the flexible wiring board as a unit for transmitting an electrical signal to a recording element substrate.
2. Description of the Related Art
In a typical flexible wiring board, a copper wire is patterned on a substrate (generally, referred to as “base film”) having flexibility, and the patterned copper wire is covered with a film material (generally, referred to as the “cover film”). That is, the flexible wiring board has a sandwich structure in which the copper wire is sandwiched and protected by the base film and the cover film. Therefore, there may also be a case where the base film and the cover film are grouped together and referred to as a protective film.
The flexible wiring board with such a structure has an advantage of being capable of enhancing the degree of freedom of a layout of installation of electronic components and substrates, and has been in widespread use for various kinds of small to large electronic instruments or the like. For instance, Japanese Patent Application Laid-Open No. 2003-72042 (corresponding to U.S. Pat. No. 6,802,595) discusses a flexible wiring board used for an ink jet recording head. In this flexible wiring board, a width thereof increases between two bending parts, and correspondingly to this increase, a pitch of a plurality of wires provided on the bending parts also increases. Further, in accordance with an increase in width of the flexible wiring board, the width of each wire itself becomes wider in the vicinity of the bending parts.
However, when the flexible wiring board is flexed and used in such a state, there may be a case where a bending stress in a flexing part causes the failure of an electrically connecting section between the flexible wiring board and another member. To solve this problem, Japanese Patent Application Laid-Open No. 10-76647 discloses a flexible wiring board in which the width of a bending part of the flexible wiring board is made to be 40% to 70% of that of the area of a substrate on which electrodes are disposed.
Further, to solve a similar problem, Japanese Patent Application Laid-Open No. 2004-235321 discloses a flexible wiring board in which a notch part is formed at the end in a bending position. This configuration reduces a resilient stress against bending (a force acting to return a flexible member to original after bending), thereby allowing the bending to be easily performed in a correct position. Further, as shown in FIG. 8, Japanese Patent Application Laid-Open No. 2004-235321 discloses a configuration in which a pitch t2 of a wiring pattern 302 in a predetermined bending position 301 is made narrower as compared with a pitch t1 in a position other than the predetermined bending position 301. However, this configuration is employed for the purpose of securing an area for forming a notch part 303 at the end of the flexible wiring board corresponding to the predetermined bending position 301. Also, similarly, as shown in FIG. 8, Japanese Patent Application Laid-Open No. 2004-235321 discloses a configuration in which a distance s2 between the end of the flexible wiring board 300 and the edge of the closest wiring pattern 302 in the predetermined bending position 301 is made longer as compared with a distance s1 between them in a position other than the predetermined bending position 301. However, this configuration is also employed for the purpose of increasing an area of the end portion of the flexible wiring board 300 in the predetermined bending position 301 and easily forming the notch part 303. For this reason, the size of the distance s1 is similar to that of a distance s3 in the notch part 303 even on a predetermined bending line 304 at which the largest bending stress acts on the flexible wiring board 300. The predetermined bending line means a line supposed to be formed on the flexible wiring board 300 when the flexible wiring board 300 is bent in the predetermined bending position 301.
As described above, in the flexible wiring board having a feature allowing the flexible wiring board to be relatively freely flexed, there has also been recognition of a problem caused by the flexible wiring board returning to an original shape after bending.
Another problem which may arise by bending a flexing part at a right angle or at a small bend radius close to a right angle is that there is a possibility that a large difference of stress occurs between the compression side and the extension side of a base film and a cover film. As a result, the base film and the cover film may be locally peeled off. Then, a problem may arise in that, if the base film and the cover film are peeled off, lifting between the films occurs in a peeled area or in the vicinity thereof, and a space is generated between the wire and the film, thus decreasing the function of protecting the wire. Further, if the above-described space spreads to the end portion of the flexible wiring board, a liquid containing an electrolyte may enter the space between the protective films from the outside thereof, thus causing various problems such as a short circuit, a current leakage, corrosion of the wire or the like.